Non-conservative behaviors of chromophoric dissolved organic matter in a turbid estuary: Roles of multiple biogeochemical processes

Chromophoric dissolved organic matter (CDOM) may show notable non-conservative behaviors in many estuaries due to a variety of biogeochemical processes. The partition between CDOM and chromophoric particulate organic matter (CPOM) was examined in the Jiulong Estuary (China) using absorption and fluorescence spectroscopy, which was also compared with microbial and photochemical degradations. The absorption coefficient of water-soluble CPOM (aCPOM(280)) at ambient Milli-Q water pH (6.1) ranged from 0.11 to 7.94 m−1 in the estuary and was equivalent to 5–101% of CDOM absorption coefficient. The aCPOM(280) correlated significantly with the concentration of suspended particulate matter and was highest in the bottom water of turbidity maximum zone. Absorption spectral slope (S275–295) and slope ratio (SR) correlated positively with salinity for both CPOM and CDOM, suggesting decreases in the average molecular weight with increasing salinity. The adsorption of CDOM to re-suspended sediments (at 500 mg L−1) within 2 h was equivalent to 4–26% of the initial aCDOM(280). The adsorption of CDOM to particles was less selective with respect to various CDOM constituents, while the microbial degradation resulted decreases in S275–295 and SR of CDOM and preferential removal of protein-like components. The partition between CPOM and CDOM represented a rapid and important process for the non-conservative behavior of CDOM in turbid estuaries.